Coaxial cables comprise inner and outer conductors disposed in spaced concentric relationship with a nonconductive insulating material disposed uniformly therebetween. Coaxial cables typically are used to carry radio frequency or microwave frequency electrical signals.
Connectors are provided to electrically join the inner and outer conductors of a coaxial cable to other electrical components such as a circuit board, a microstrip, a coplanar wave guide or to another cable. It is important for the coaxial cable connector to make secure electrical and mechanical connections to both the inner and outer conductors of the cable and to maintain substantial coaxial symmetry across the connector. Significant variations in this symmetry can degrade the signal transmitted through the connector.
Coaxial cable connectors comprise substantially concentric inner and outer conductors for electrically and mechanically engaging the inner and outer conductors of a cable and/or the inner and outer conductive portions of another component. The cylindrical configuration of the outer conductor in the connector substantially ensures its required coaxial symmetry across the connector. However, the inner conductor has a radial dimension that is very small compared to its axial length. As a result, coaxial connectors typically are provided with support means surrounding the inner conductor, and supporting the inner conductor relative to the outer conductor. The support means for a coaxial connector typically comprises a bead formed from an electrically nonconductive material. The bead is constructed to be disposed generally concentrically between the inner and outer conductors. For example, most prior art beads are substantially annular in configuration and comprise a centrally disposed through aperture for surrounding and engaging the center conductor of the coaxial connector. Extreme precision is required in forming or machining the support bead. Any nonconcentricity or any manufacturing defects, such as burrs, nicks or the like, will invariably affect the electrical performance of the connector by causing losses to occur under certain operating conditions and/or at certain frequency ranges.
Dielectric support beads create an impedance which is proportional to the dielectric constant of the support bead. The dielectric constant, in turn, is indicative of the radio frequency at which the connector is capable of operating. In particular, higher operating frequencies generally can be obtained by a support bead having a low dielectric constant. Most subminiature coaxial connectors (SMA connectors) in the prior art have operated at or below 26 GHz.
It is desirable to provide coaxial connectors that can operate at frequency ranges higher than 26 GHz. As noted above, one factor affecting the performance of the coaxial connector at very high frequency ranges is the dielectric constant of the support bead. In particular, a dielectric support bead with a low dielectric constant helps to achieve a coaxial connector that can operate at higher frequency ranges. However, the options for improved engineering of the dielectric support bead are somewhat limited in view of the fact that the overall dimension of the dielectric support bead generally must be in accordance with established military specifications, which are intended to ensure compatibility of connectors.
One possible approach for reducing the dielectric constant of the support bead, and thereby increasing the frequency at which the associated connector can operate is to reduce the total amount of dielectric material in the support bead. This is somewhat difficult to achieve, however, in view of the overall external dimensional requirements of the support bead and in view of the importance of manufacturing a support bead that will keep contaminants out of the connector, such as gold flakes that may be delaminated from the plated contact surfaces of certain terminals.
One especially effective dielectric support bead for achieving the above described objectives is shown in U.S. Pat. No. 4,718,864 which issued to James R. Flanagan on Jan. 12, 1988 and which is assigned to the assignee of the subject invention. In particular, U.S. Pat. No. 4,718,864 shows an injection molded dielectric support bead having a central through aperture for supporting the center conductor of the connector, and further having an array of apertures extending through a first end face of the bead and disposed concentrically about the central through aperture thereof. Each aperture of the array of apertures in the bead of U.S. Pat. No. 4,718,864 extends a major portion of the distance through the support bead, and is aligned parallel to the central through aperture thereof. The support bead shown in U.S. Pat. No. 4,718,864 further comprises annular undercuts in the respective opposed axial ends. The bead shown in U.S. Pat. No. 4,718,864 provides adequate structural support between the inner and outer conductors of the coaxial connector, while further providing a substantially reduced amount of dielectric material in the bead. This reduced amount of dielectric material enables the overall dielectric constant of the bead to be reduced, and as a result, substantially higher operating frequencies are achievable with the associated connector. It has been found that connectors employing the dielectric support bead shown in U.S. Pat. No. 4,718,864 can operate successfully at 40 GHz.
Despite the many efficiencies of the bead described in U.S. Pat. No. 4,718,864, it is considered desirable to provide a bead that has even better performance characteristics over a broader range of environmental conditions. For example, certain coaxial electrical connectors, such as those used in military, aeronautical, and aerospace applications are subjected to extremely broad ranges of temperature. In particular, coaxial connectors may be employed in an environment having temperatures as low as -65.degree. C. or temperatures as high as 165.degree. C. In many known connectors these extreme temperature variations, and especially the high ranges of temperature can cause physical changes in the dielectric bead structure which will permanently degrade the signal carrying performance of the connector even after the connector has returned to less severe temperatures. Although the bead shown in U.S. Pat. No. 4,718,864 performs well under a broad range of conditions, it is desirable to provide a bead that exhibits less signal degradation after extreme environmental temperature cycling. It has also been found that although the particular bead construction shown in U.S. Pat. No. 4,718,864 performs extremely well, the highly precise apertures formed therein are difficult and costly to manufacture with the required high degree of precision.
Other coaxial connectors with support beads that have attempted to ensure high frequency signal transmission include: U.S. Pat. No. 3,229,234 which issued to Lattanzia; U.S. Pat. No. 3,492,605 which issued to Ziegler; U.S. Pat. No. 4,431,255 which issued to Banning; German Pat. No. 3033690; French Pat. No. 664,271 and British Pat. No. 1,490,421. These prior art references are considered to be less desirable in high frequency signal carrying environments than the connector and molded dielectric support bead therefor shown in U.S. Pat. No. 4,718,864. Furthermore, these prior art structures provide no teaching that would enable the connector and support bead to function in environments of extreme ranges of temperature variation.
In view of the above, it is an object of the subject invention to provide a dielectric support bead for a coaxial connector that can perform reliably at extremely high frequencies.
It is another object of the subject invention to provide a dielectric support bead with substantially invariable operating characteristics over a very broad range of temperatures.
It is an additional object of the subject invention to provide a dielectric support bead -or a coaxial connector that is substantially insensitive to frequent extreme changes in temperature.
A further object of the subject invention is to provide a dielectric support bead with a low dielectric constant and with an ability to keep contaminants out of the connector.
Still another object of the subject invention is to provide a coaxial connector having a dielectric support bead that enables the transmission of high frequencies signals even after subjection to extreme ranges of temperature cycling.